Currently, there are two main technologies that are used for image sensing and light detection in a camera: pixels based on the charge-coupled devices (CCD) and complementary metal-oxide semiconductor (CMOS) based active pixel sensors (APS). Both technologies transduce light into an electrical output signal in two steps: first converting the received amount of light into a corresponding number of electrons via photoelectric effect observed in semiconductors, and second, processing these photo-generated electrons in order to produce a measurable electrical signal. Each technology has different strengths and weaknesses, and they differentiate from each other mainly in the second step. CCDs are inherently charge-mode devices, whereas APS type sensors may use different methods to convert photo-generated carriers into an electrical signal.
In a CCD sensor, many signal processing functions are undertaken outside the sensor area, as CCD pixels and the signal electronics are made by different technologies, whereas a CMOS sensor incorporates the processing units such as analog-to-digital (AD) converters, sample-and-hold circuits, amplifiers, etc. on the same chip. Therefore, compared to CCD sensors, CMOS sensors are more integration friendly, thus power and cost effective. However, the proximity between the circuitry and the active area with the pixels where light absorption takes place in a CMOS sensor may induce noise related problems. CCD was the predominant technology which enabled diverse consumer applications since their inception in 1970s and offered better light sensitivity and less noise compared to CMOS sensor in the past. With the recent technological advancements, such as back side illumination (BSI) CMOS sensors caught up with CCD technology in terms of image quality and they rapidly took up the automotive and mobile applications.
FIG. 1 shows the state of the art array configuration of CMOS image sensor (CIS) technology. Each APS pixel requires three to four (3-4) transistors for cell selectivity in an array, reset and charge transfer functions and a photodiode as the light sensitive device. By using a shared buffer and row selection transistors among eight (8) pixels, the effective number of transistors per pixel is reduced down to a ratio 1.375. By doing so, higher fill factors are achieved.
However, despite all these advances in the field of light and photon detection, pixel and image sensors, there is still great need to improve the current CIS performance in terms of sensitivity, speed and the dynamic range at scaled technology nodes.